Simple decoupling of a multi-element RF coil, enabling also detuning and matching functionality

ABSTRACT

A coil ( 36 ) includes coil elements ( 38   1   , 38   2   , . . . , 38   n ). The coil ( 36 ) can transmit radio frequency excitation pulses into an examination region ( 14 ) and/or receive responsive radio frequency pulses from the examination region ( 14 ). A compensation network ( 42 ) includes decoupling segments ( 98 ), which each has a selected electrical length at least of a quarter wavelength (λ/4) and is electrically coupled to an associated coil element ( 38   1   , 38   2   , . . . , 38   n ) and a reactive network ( 100 ). The compensation network ( 42 ) at least compensates coupling between the coil elements ( 38   1   , 38   2   , . . . , 38   n ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 60/745,459 filed Apr. 24, 2006, which is incorporated herein byreference.

The present application relates to the magnetic resonance arts. It findsparticular application in magnetic resonance imaging coils and scanners,and will be described with particular reference thereto. More generally,it finds application in magnetic resonance systems for imaging,spectroscopy, and so forth.

Magnetic resonance imaging (MRI) apparatus is commonly used for theexamination of patients. In MRI, RF coils are used to generate B₁ fieldswithin the imaging subject for exciting the nuclear spins and to detectsignals from the nuclear spins.

In some multi-channel transmit/receive MRI systems, one of a pluralityof transmitting units is assigned to each RF coil or coil segment andprovided for independently adjusting the amplitude and/or the phaseand/or the shape of the RF waveform to be transmitted; while one of aplurality of receiving units is assigned to each RF coil or coilsegment. More specifically, independent amplitudes and/or the phasesand/or the shapes of the RF waveforms to be transmitted are used tocompensate for dielectric resonances in examination objects or to exciteand optimize a desired excitation pattern or to shorten the transmitpulse length such as in Transmit SensE.

Locating several RF transmitters in close proximal alignment causesmutual coupling between the antenna or coil elements. The phases andamplitudes of the currents in coupled antenna elements becomeinterrelated. Power is exchanged among the RF transmit channels.

One method to compensate for mutual coupling is to use passivedecoupling networks. Passive decoupling methods are applicable in auseful manner for a limited number of coils since the determination ofthe capacitive and/or inductive elements becomes rather difficult for alarge number of channels. In addition, a decoupling and matching networkcan only be determined and assembled for the expected standard load,which is not necessarily the actual load. At higher fields, smallchanges in load can have a significant effect on the decoupling ofelements. Another problem in the passive decoupling networks includesthe presence of parasitic capacitances and inductances of theconnectors, which might cause undesired resonances.

The present application provides new and improved methods andapparatuses which overcome the above-referenced problems and others.

In accordance with one aspect, a coil system is disclosed. A coilincludes coil elements. The coil at least one of transmits radiofrequency excitation pulses into an examination region and receivesresponsive radio frequency pulses from the examination region. Acompensation network includes decoupling segments, which each has aselected electrical length at least of a quarter wavelength (or anequivalent) and is electrically coupled to an associated coil elementand a reactive network which includes capacitors and/or inductors. Thecompensation network at least compensates magnetic coupling between thecoil elements.

In accordance with another aspect, a magnetic resonance system isdisclosed. A main magnet generates a main magnetic field through anexamination region. A plurality of RF transmitters generates RFresonance excitation pulses at a resonance frequency of selected dipolesin the examination region. A plurality of RF receivers receives anddemodulates resonance signals from dipoles in the examination region. Aplurality of RF coil elements is disposed adjacent the examinationregion. A plurality of effective quarter wavelength cables, eachincluding an RF cable conductor, is connected between the coil elementsand the reactive network. At least one of the transmitters and/orreceivers can be connected to the coil via the cables.

One advantage is that each coil element is decoupled from the other coilelements individually.

Still further advantages of the present invention will be appreciated tothose of ordinary skill in the art upon reading and understand thefollowing detailed description.

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1 is a diagrammatic illustration of a magnetic resonance imagingsystem;

FIG. 2 is a diagrammatic illustration of a TEM coil;

FIG. 3 is a diagrammatic illustration of a coil arrangement including aTEM coil and a compensation network;

FIG. 4 is a diagrammatic illustration of a coil arrangement including abirdcage coil and a compensation network;

FIG. 5 is a diagrammatic illustration of another coil arrangementincluding a birdcage coil and a compensation network; and

FIG. 6 is a diagrammatic illustration of a coil arrangement includingloop resonators and a compensation network.

With reference to FIGS. 1 and 2, a magnetic resonance imaging system 8includes a scanner 10 including a housing 12 defining an examinationregion 14, in which is disposed a patient or other imaging subject 16 ona patient support or bed 18. A main magnet 20 disposed in the housing 12generates a main magnetic field B₀ in the examination region 14.Typically, the main magnet 20 is a superconducting magnet surrounded bycryo shrouding 24; however, a resistive or permanent main magnet canalso be used. Various B₀ magnetic fields are contemplated such as 3T atwhich protons has a resonance frequency of 128 MHz or 7T at whichprotons have a resonance frequency of 300 MHz. Magnetic field gradientcoils 30 are arranged in or on the housing 12 to superimpose selectedmagnetic field gradients on the main magnetic field within theexamination region 14. An RF coil system or arrangement 34 with asurrounding shield 40 is disposed about the examination region 14. Thecoil system 34 includes one or more RF coils 36 which each includes aplurality of radio frequency coil elements, segments, loops, or rungs 38which each might have a different size and position. Although a localhead coil is illustrated, it is to be appreciated that whole body coils,local surface coils and the like are also contemplated. The coil 36 maybe a TEM coil, a birdcage resonator, an arrangement of loop resonators,or the like. In the exemplary embodiment, the coil 36 includes aplurality (n) of elements or segments 38 ₁, 38 ₂, . . . , 38 _(n)positioned around or in the intended volume of examination. The coil 36is, for example, circularly cylindrical, but, of course, might haveother geometries, such as an elliptic cross-section, semi-circularcross-section, semi-elliptical cross-section, and the like. As describedin detail below, a compensation network 42 including cable assemblies,each of a selected electrical length, is coupled to the coil 36 and areactive network to at least decouple the coil elements 38 from eachother.

With continuing reference to FIG. 1, a magnetic resonance imagingcontroller 48 operates magnetic field gradient controllers 50 coupled tothe gradient coils 30 to superimpose selected magnetic field gradientson the main magnetic field in the examination region 14, and alsooperates a plurality (e.g. n) radio frequency transmitters 52 eachcoupled by a transmit/receive switch 54 to an individual radio frequencycoil element 38 ₁, 38 ₂, . . . , 38 _(n) or a subset of the segments toinject selected radio frequency excitation pulses at about the magneticresonance frequency into the examination region 14 for imaging. Theradio frequency transmitters 54 are individually controlled and can havedifferent phases and amplitudes. The radio frequency excitation pulsesexcite magnetic resonance signals in the imaging subject 16 that arespatially encoded by the selected magnetic field gradients. Stillfurther, the imaging controller 50 operates a plurality (e.g. n) radiofrequency receivers 56 that each is individually controlled andconnected with the individual coil element 38 ₁, 38 ₂, . . . , 38 _(n)of the coil system 34 by the transmit/receive switch 54 to demodulatethe received and spatially encoded magnetic resonance signals. In theembodiments, where a transmit-only and/or a receive-only coil is used,the transmit/receive switch is omitted. Such coil is detuned in one of areceive or transmit phase. A coil that is used for both transmit andreceive does not need to be switched off or detuned except for anapplication, where it remains inside the scanner while other coils areused. The received spatially encoded magnetic resonance data is storedin a magnetic resonance or MR data memory 60.

A reconstruction processor, algorithm, device, or other means 62reconstructs the stored magnetic resonance data into a reconstructedimage of the imaging subject 16 or a selected portion thereof lyingwithin the examination region 14. The reconstruction processor 62employs a Fourier transform reconstruction technique or other suitablereconstruction technique that comports with the spatial encoding used inthe data acquisition. The reconstructed image is stored in an imagememory 64, and can be displayed on a user interface 66, transmitted overa local area network or the Internet, printed by a printer, or otherwiseutilized. In the illustrated embodiment, the user interface 66 alsoenables a radiologist or other user to interface with the imagingcontroller 50 to select, modify, or execute imaging sequences. In otherembodiments, separate user interfaces are provided for operating thescanner 10 and for displaying or otherwise manipulating thereconstructed images.

The described magnetic resonance imaging system 8 is an illustrativeexample. In general, substantially any magnetic resonance imagingscanner can incorporate the disclosed radio frequency coils. Forexample, the scanner can be an open magnet scanner, a vertical borescanner, a low-field scanner, a high-field scanner, or so forth. In theembodiment of FIG. 1, the coil 36 is used for both transmit and receivephases of the magnetic resonance sequence; however, in other embodimentsseparate transmit and receive coils may be provided, one or both ofwhich may incorporate one or more of the radio frequency coil designsand design approaches disclosed herein.

With particular reference to FIG. 2, the example illustrated radiofrequency body coil is a TEM coil 36 (not to scale) which includes aplurality of elements 38 ₁, 38 ₂, . . . , 38 _(n). The elements 38 ₁, 38₂, . . . , 38 _(n) in this embodiment are arranged in parallel to oneanother and the B₀ field and surrounding the examination region 14. Inthe illustrated coil 36, the elements 38 ₁, 38 ₂, . . . , 38 _(n)include printed circuit strips disposed on an electricallynon-conducting generally cylindrical substrate 72. The RF shield 40extends spherically around the coil 36 and may be a conductive layer onan opposite face of the printed substrate 72 or a separate structure.Each element is connected to the RF shield 40, for example, via aresonance capacitor.

With reference to FIG. 3, the compensation network 42 includes cables orcable assemblies 98 which each is characterized by a selected electricallength and electrically coupled to an associated element 38 ₁, 38 ₂, . .. , 38 _(n) and a reactive network 100. For example, each cable 98 ischaracterized by an electrical length of a quarter wavelength (λ/4) atthe resonance frequency or an electrical length of a quarter wavelength(λ/4) with an addition of an integer times half wavelength (λ/4+k λ/2)at the resonance frequency. Other elements or circuits that are theelectrical equivalent of a quarter wavelength cable (λ/4) or a quarterwavelength (λ/4) with an addition of an integer times half wavelength(λ/4+k λ/2) are contemplated. In a 7T scanner, for example, whereprotons have a resonance frequency of 300 MHz, a quarter wave line wouldhave a length of λ/4=25 cm, if the line's relative dielectric constantis equal to unity. More specifically, a first connection point 102 of aline or RF conductor 104 is electrically coupled to a first connectionpoint 106 of the associated element 38 ₁, 38 ₂, . . . , 38 _(n). Asecond connection point 108 of the line 104 is electrically coupled tothe reactive network 100. The reactive network 100 includes a pluralityof reactive elements, such as capacitors and/or inductors, values ofwhich are determined such that at least each two elements 38 ₁, 38 ₂, .. . , 38 _(n) are decoupled from each other. In the example of FIG. 3,capacitors 120, 122 are coupled between corresponding pairs of nearestneighboring elements 38 ₁ and 38 ₂, 38 ₂ and 38 ₃ to decouple nearestneighbors. Capacitors 124 are coupled between next, nearest neighboringelements 38 ₁ and 38 ₃ to decouple next, nearest neighbors. Additionalreactive elements can be provided to decouple from more remote elements.Of course, it is contemplated that the reactive network 100 can includea variety of compensating reactive elements coupled in a variety ofconfigurations.

Each cable assembly 98 includes an associated cable shield or shield orshield conductor 130 connected to a second connection point 132 of eachassociated element 38 ₁, 38 ₂, . . . , 38 _(n) and the RF shield 40which might be connected to a ground point 134 of the reactive network100.

In one embodiment, a switching device 140 such as PIN diode is coupledbetween the second connection point 108 of the conductor 104 and groundpoint 134 for detuning the element 38 ₁, 38 ₂, . . . , 38 _(n) bygrounding an associated cable 98. When a body coil is used for transmitand a local coil is used for receive, the switching devices 140 on thelocal coil are controlled to detune the local coil on transmit, e.g. theswitching diodes are forward biased. Similarly, the switching devices140 on the body coil can detune the body coil during receive.Optionally, additional tuning elements can be connected in parallel tothe switching devices 140 to tune the coil elements.

With reference to FIG. 4, in a birdcage embodiment, the radio frequencycoil 36 includes a plurality of elements 38 ₁, 38 ₂, . . . , 38 _(n) inthe form of rungs which are arranged in parallel to one another tosurround the examination region 14. The elements 38 ₁, 38 ₂, . . . , 38_(n) are connected to first and second end rings 150, 152 which providea return current path. Each element 38 ₁, 38 ₂, . . . , 38 _(n) is splitinto first and second portions 154, 156 to expose the first and secondconnection points 106, 132 to be coupled to the associated cableassemblies 98. Similarly to the embodiment of FIG. 3, each cableassembly 98 is characterized by a selected electrical length. The firstconnection point 102 of each conductor 104 is electrically coupled tothe first connection point 106 of the associated element 38 ₁, 38 ₂, . .. , 38 _(n). The second connection point 108 of the conductor 104 iselectrically coupled to the reactive network 100. The reactive network100 includes a plurality of reactive elements, such as capacitors and/orinductors, values of which are determined such that at least twoelements 38 ₁, 38 ₂, . . . , 38 _(n) are decoupled from each other. Inthe example of FIG. 4, the capacitors 120, 122 are coupled betweencorresponding pairs of neighboring nearest elements 38 ₁ and 38 ₂, 38 ₂and 38 ₃ to decouple nearest neighbors. In one embodiment, the nearestneighbors, e.g. the coil elements 38 ₁ and 38 ₂, 38 ₂ and 38 ₃, aredecoupled from each other by selecting an appropriate ratio betweencapacitors in the first and second end rings 150, 152 and resonancecapacitors in the elements 38 ₁, 38 ₂, . . . , 38 _(n), illustrated aslumped capacitors 160, 162, 164. The capacitor 124 is coupled betweennext, nearest neighboring elements 38 ₁ and 38 ₃ to decouple next,nearest neighbors. Of course, it is contemplated that the reactivenetwork 100 can include a variety of compensating reactive elementscoupled in a variety of configurations. The cable shields 130 areconnected with the ground point 134 of the reactive network 100. Ofcourse, it is contemplated that the cable shields 130 can be connectedto different ground planes such as a coil ground point.

With reference to FIG. 5, the first connection point 102 of eachconductor 104 is connected to the first connection point 106 of theassociated element 38 ₁, 38 ₂, . . . , 38 _(n). In this embodiment, thefirst connection point 106 of the coil element is disposed at about aconnection point between the element 38 ₁, 38 ₂, . . . , 38 _(n) and thefirst end ring 150. The second connection point 108 of the conductor 104is electrically coupled to the reactive network 100. The shield 130 ofeach cable assembly 98 is connected between the RF screen 40 of the coil36 and the ground point 134.

With reference to FIG. 6, in a surface coil embodiment, each element 38₁, 38 ₂, . . . , 38 _(n) is a loop. In one embodiment, the loop includesresonance capacitors illustrated as lumped capacitors 162. Each loop isopened to expose the first and second connection points 106, 132 to becoupled to the cable assembly 98. Similar to the embodiment of FIG. 3,the first connection point 102 of the conductor 104 is electricallycoupled to the first connection point 106 of the associated loop 38 ₁,38 ₂, . . . , 38 _(n). The second connection point 108 of the conductor104 is electrically coupled to the reactive network 100. The reactivenetwork 100 includes a plurality of reactive elements, such ascapacitors and/or inductors, values of which are determined such thateach loop 38 ₁, 38 ₂, . . . , 38 _(n) is decoupled from other loops. Inthe example of FIG. 6, the capacitors 120, 122 are coupled betweencorresponding pairs of nearest neighboring loops 38 ₁ and 38 ₂, 38 ₂ and38 ₃ to decouple nearest neighbors. The capacitor 124 is coupled betweennext, nearest neighboring loops 38 ₁ and 38 ₃ to decouple next, nearestneighbors. Of course, it is contemplated that the reactive network 100can include a variety of compensating reactive elements coupled in avariety of configurations.

In one embodiment, the cable assembly 98 is used to match the impedancesof the coil elements to the impedance(s) of feeding or transmittingline(s) 170. This can be realized by choosing the appropriate lineimpedances for the cables 98. It is also contemplated that the feedingline 170 can be connected directly to the coil 36, optionally, via amatching network.

To explain the theory of the decoupling described above:

Generally, in a transmission line, which extends from z=−∞ to z=+∞, thevoltage U (z) and current I (z) are z dependent, where z is theposition. As used in this document, the underlined values are peakphasors, e.g. I(t)=real(I*exp(jwt)). With an apostrophe describing thedeviation in space z, the differential equations can be derived for thevoltage U and current I (in z-direction):U′=−Z′I   (1)I′=−Y′U   (2)

-   where Z′ is differential impedance of the transmission line:    Z′=R′+jωL′-   and Y′ is differential admittance of the transmission line:    Y′=G′+jωC′

The wave equations are derived as:U″=Z′Y′U  (3)I″=Z′Y′I  (4)

The general solutions for the voltage and current are:U=U ₁ e ^(−γz) +U ₂ e ^(+γz)  (5)I=1/Z ₀( U ₁ e ^(−γz) −U ₂ e ^(+γz))  (6)

-   where Z₀ is the wave impedance; and-   γ is the wave number of the transmission line.

The wave impedance Z₀ is a ratio of voltage and current for a travelingwave in one direction is:Z ₀=√(Z′/Y′)  (7)

-   where Z′ is differential impedance of the transmission line, and-   Y′ is differential admittance of the transmission line.

The wave number of the transmission line is related to the speed anddamping of the transmission and is:γ=√(Z′Y′)  (8)

-   where Z′ is differential impedance of the transmission line, and-   Y′ is differential admittance of the transmission line.

Assuming that the transmission line extends from −∞ the position z equalto 0 with the boundary condition U=ZI, given by the impedance Z, for theposition z equal to 0, equations (5) and (6) can be written as:

$\begin{matrix}{r = {\frac{{\underset{\_}{U}}_{2}}{{\underset{\_}{U}}_{1}} = {{- \frac{{\underset{\_}{I}}_{2}}{{\underset{\_}{I}}_{1}}} = \frac{Z - Z_{0}}{Z + Z_{0}}}}} & (9)\end{matrix}$

-   where Z₀ is the wave impedance; and-   r is a reflection factor which provides the relation of the waves in    the two directions, e.g. positive and negative z-directions, which    is given by the impedance at the end of the transmission line and    the wave impedance.

For high frequencies, it is more convenient to use wave amplitudes,which are related to the power. For example, for the transmission linesof the TEM coil, the wave amplitudes a, b of the waves in the positiveand negative z-direction are determined as:a (z=0):= U ₁/√(2Z ₀)  (10)b (z=0):= U ₂/√(2Z ₀)  (11)

-   where z is the position,-   Z₀ is the wave impedance which is assumed to be real,-   a is the amplitude of the wave traveling in the positive z-direction    at the position z=0,-   b is the amplitude of the wave traveling in the negative z-direction    at the position z=0,-   U ₁ is the voltage of the wave traveling in the positive z-direction    in the transmission line at the position z=0, and-   U ₂ is the voltage of the wave traveling in the negative z-direction    in the transmission line at the position z=0.

As observed from equations (10) and (11), the reflection coefficient rdefined in equation (9) is the ratio of b to a. For any position z, thefirst and second wave amplitudes a, b can be expressed as:a (z):=1/√(8Z ₀)( U (z)+Z ₀ I (z))  (12)b (z):=1/√(8Z ₀)( U (z)+Z ₀ I (z))  (13)

-   where z is the position,-   Z₀ is the wave impedance,-   a(z) is the amplitude of the first wave at the position z,-   b(z) is the amplitude of the second wave at the position z,-   U(z) is the transmission line voltage at the position (z), and-   I(z) is the transmission line current at the position (z).

The first and second wave amplitudes a and b can also describe a linearN-port device. In this case, the first and second wave amplitudes a andb become vectors. The transmission line impedance for each port can bewritten in as a vector:

-   -   {right arrow over (Z₀)},

The vectors of the first and second wave amplitudes for each port can bepresented as:

$\begin{matrix}{\overset{\rightarrow}{\underset{\_}{a}} = {\frac{1}{\sqrt{8}}{diag}\mspace{11mu}{{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}\left( {\overset{\rightarrow}{\underset{\_}{U}} + {{diag}\;{\overset{\rightarrow}{Z}}_{0}\overset{\rightarrow}{\underset{\_}{I}}}} \right)}}} & (14) \\{\overset{\rightarrow}{\underset{\_}{b}} = {\frac{1}{\sqrt{8}}{diag}{{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}\left( {\overset{\rightarrow}{\underset{\_}{U}} - {{diag}\;{\overset{\rightarrow}{Z}}_{0}\overset{\rightarrow}{\left. \underset{\_}{I} \right)}}} \right.}}} & (15)\end{matrix}$

-   where diag Z₀ ^(−1/2) is a diagonal matrix of the inverse square    roots of the wave impedances,-   {right arrow over (a)} is the vector of the first wave amplitudes,    traveling into the device,-   {right arrow over (b)} is the vector of the first wave amplitudes,    coming out of the device,-   {right arrow over (U)} is the vector of the voltages at the ports of    the device,-   {right arrow over (I)} is the vector of the currents flowing into    the device,-   {right arrow over (Z)}₀ is the vector of the wave impedances, and-   diag{right arrow over (Z)}₀ is the diagonal matrix build from the    elements of {right arrow over (Z₀)}.

By solving equations (14) and (15), the values for voltage and currentare:

$\begin{matrix}{\overset{\rightarrow}{\underset{\_}{U}} = {\sqrt{2}{diag}{{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}\left( {\overset{\rightarrow}{\underset{\_}{a}} + \overset{\rightarrow}{\underset{\_}{b}}} \right)}}} & (16) \\{\overset{\rightharpoonup}{\underset{\_}{I}} = {\sqrt{2}{diag}\;{{\overset{\rightharpoonup}{Z}}_{0}^{- \frac{1}{2}}\left( {\overset{\rightharpoonup}{\underset{\_}{a}} - \overset{\rightharpoonup}{\underset{\_}{b}}} \right)}}} & (17)\end{matrix}$

A linear device can be presented by an impedance matrix, admittancematrix or scattering matrix accordingly expressed in equations (18),(19) and (20):{right arrow over (U)}=Z{right arrow over (I)}  (18){right arrow over (I)}=Y{right arrow over (U)}  (19){right arrow over (b)}=S{right arrow over (a)}  (20)

-   where Z is the impedance matrix of the linear device,-   Y is the corresponding admittance matrix,-   S is the corresponding scattering matrix.

The relationship between Z and Y is given by inversion. The scatteringmatrix S is derived from the equations (18)-(20) using equations(14)-(15) and (16)-(17):

$\begin{matrix}{Z = {{diag}{{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}\left( {1 + S} \right)}\left( {1 - S} \right)^{- 1}{diag}{\overset{\rightarrow}{Z}}_{0}^{1/2}}} & (21) \\{S = {{diag}\;{{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}\left( {Z - {{diag}{\overset{\rightarrow}{Z}}_{0}}} \right)}\left( {Z + {{diag}\;{\overset{\rightarrow}{\underset{\_}{Z}}}_{0}}} \right)^{- 1}{diag}\;{\overset{\rightarrow}{Z}}_{0}^{1/2}}} & (22)\end{matrix}$

Equation (22) is a generalized formulation of the reflection factor r ofequation (9).

$\begin{matrix}{Y = {{diag}{{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}\left( {1 - S} \right)}\left( {1 + S} \right)^{- 1}{d{iag}}\mspace{14mu}{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}}} & (23) \\{S = {{diag}{{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}\left( {1 + {{diag}{\overset{\rightarrow}{Z}}_{0}Y}} \right)}\left( {1 - {{diag}\;{\overset{\rightarrow}{Z}}_{0}Y}} \right)^{- 1}{d{iag}}{\overset{\rightarrow}{Z}}_{0}^{\frac{1}{2}}}} & (24) \\{\mspace{14mu}{= {{diag}{{\overset{\rightarrow}{Z}}_{0}^{\frac{1}{2}}\left( {{{diag}{\overset{\rightarrow}{Z}}_{0}^{- 1}} + Y} \right)}\left( {{{diag}{\overset{\rightarrow}{Z}}_{0}^{- 1}} - Y} \right)^{- 1}{diag}{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}}}} & (25)\end{matrix}$

A quater wave line has the following scattering matrix

$\begin{matrix}\begin{bmatrix}0 & {- j} \\{- j} & 0\end{bmatrix} & (26)\end{matrix}$

A system of N quarter wave lines from port 1, . . . , N to port N+1, . .. , 2N results in the scattering matrix S_(λ/4):

$\begin{matrix}{S_{\lambda,4} = \begin{bmatrix}0 & {- j} \\{- j} & 0\end{bmatrix}} & (27)\end{matrix}$where j denotes a diagonal matrix of j=√−1.

Connecting ports N+1, . . . , 2N to a device which has a scatteringmatrix S_(d) results in a scattering matrix (related to the notconnected ports):S=−S _(d)  (28)

If the device which has a scattering matrix S_(d), and an admittancematrix Y_(d) is transformed by such a set of quarter wave lines, theresulting impedance matrix is:

$\begin{matrix}\begin{matrix}{Z = {{diag}{{\overset{\rightarrow}{\underset{\_}{Z}}}_{0}^{\frac{1}{2}}\left( {1 + S} \right)}\left( {1 - S} \right)^{- 1}{diag}{\overset{\rightarrow}{\underset{\_}{Z}}}_{0}^{1/2}}} \\{= {{diag}{{\overset{\rightarrow}{Z}}_{0}^{\frac{1}{2}}\left( {1 - S_{d}} \right)}\left( {1 + S_{d}} \right)^{- 1}{diag}{\overset{\rightarrow}{Z}}_{0}^{1/2}}} \\{= {{diag}{\overset{\rightarrow}{Z}}_{0}{diag}\mspace{14mu}{{\overset{\rightharpoonup}{Z}}_{0}^{- \frac{1}{2}}\left( {1 - S_{d}} \right)}\left( {1 + S_{d}} \right)^{- 1}{diag}\mspace{14mu}{\overset{\rightarrow}{Z}}_{0}^{- \frac{1}{2}}{diag}{\overset{\rightarrow}{Z}}_{0}}} \\{= {{diag}{\overset{\rightarrow}{\underset{\_}{Z}}}_{0}Y_{d}\mspace{14mu}{diag}{\overset{\rightarrow}{\underset{\_}{Z}}}_{0}}}\end{matrix} & (29)\end{matrix}$

For example, the coil includes N elements which resonate at theoperating frequency f=ω/(2π). If each element is opened to generate aport, a N-port network can be generated. If a short is connected to portn and all the other ports are left open, then the element number nbecomes resonant at the frequency f while other elements are notoperating.

The diagonal elements of the impedance matrix Z_(coil) of the coil aredefined by:

$\begin{matrix}{Z_{{coil\_ n},n} = \left. \frac{{\underset{\_}{U}}_{{coil},n}}{{\underset{\_}{I}}_{{coil},n}} \right|_{{\underset{\_}{I}}_{{coil},k} = {0\mspace{14mu}{\forall\mspace{14mu}{k \neq n}}}}} & (30)\end{matrix}$

-   where Z_(coil) _(—) _(n,n) is the loss resistance R_(n) of the    element n:    Z_(coil) _(—) _(n,n)=R_(n)  (31)    The non diagonal elements of the impedance matrix Z_(coil) of the    coil are:

$\begin{matrix}{Z_{{coil\_ m},n} = \left. \frac{{\underset{\_}{U}}_{{coil},m}}{{\underset{\_}{I}}_{{coil},n}} \right|_{{\underset{\_}{I}}_{{coil},k} = {0\mspace{14mu}{\forall\mspace{14mu}{k \neq n}}}}} & (32)\end{matrix}$The non diagonal elements of the impedance matrix Z_(coil) of the coilsystem are given by the mutual inductance:Z_(coil) _(—) _(m,n)=jωM_(m,n)  (33)

In the ideal case of completely decoupled coil elements, the impedancematrix has only diagonal elements unequal to zero. In fact, in themajority of cases, the non-diagonal elements of the impedance matrixinclude non-zero values that have to be compensated. A compensationnetwork or device, which includes N-ports and has an impedance matrixZ_(dec), is coupled in series to the coil Z_(coil) at each port. Acombined impedance matrix Z_(Σ) for N ports of the coil system, whichincludes the coil and compensation network, can be defined for theresulting structure. In the coil system, the currents are the same inall parts, e.g. the current in the coil system is equal to the currentin the coil and current in the compensation network:{right arrow over (I)} _(Σ)={right arrow over (I)} _(coil)={right arrowover (I)} _(dec), where

-   {right arrow over (I)} _(Σ) is the current vector in the coil    system,-   {right arrow over (I)} _(coil) is the current vector in the coil,    and-   {right arrow over (I)} _(dec) is the current vector in the    compensation network.

Voltage in the coil system is equal to a sum of the voltages in the coiland compensation network:{right arrow over (U)} _(Σ) ={right arrow over (U)} _(coil) +{rightarrow over (U)} _(dec), where

-   {right arrow over (U)} _(Σ) is the voltage vector in the coil    system,

A combined impedance matrix Z_(Σ) of the coil system is equal a sum ofimpedances in the coil and compensation network:Z _(Σ) =Z _(coil) +Z _(dec)  (34)

-   Z_(Σ) the impedance matrix of the coil system,-   Z_(coil) the impedance matrix of the coil, and-   Z_(dec) the impedance matrix of the compensation system.

The coil system has to be decoupled, e.g. only the imaginary parts ofthe diagonal elements in the coil impedance matrix Z_(Σ) can be unequalto zeroes. In addition, since the combined system has to be resonant,the diagonal elements in the combined impedance matrix Z_(Σ) have to beequal to the real numbers.

The non diagonal elements of the compensation network are tuned to:Z _(dec) _(—) _(m,n) =−jωM _(m,n)  (35)

If the diagonal elements of the compensation impedance matrix of thecompensation network deviate from zero, this results in a resonancefrequency shift of the elements. This can be retuned by resonancecapacitors in each element.

To tune each element individually, the described above transmissionlines with a length of (z/2+¼) λ, where λ is the wave length inside thecable and z is an integer.

An impedance Z (or admittance Y) is transformed by such line to:

$\left. Z\rightarrow\frac{Z_{0}^{2}}{Z} \right.,\left. {{or}\mspace{14mu}{1/Y}}\rightarrow{Z_{0}^{2}Y} \right.$

This can be generalized for a set of quarter wave lines with differentline impedances:Z_(dec)=diag{right arrow over (Z)}₀{tilde over (Y)}_(dec)diag{rightarrow over (Z)}₀  (36)

In this manner, by changing a set of symmetric elements {tilde over(Y)}_(m,n) and {tilde over (Y)}_(n,m) in {tilde over (Y)}_(d) only thecorresponding elements Z_(dec,m,n) and Z_(dec,n,m) in Z_(dec) arechanged. A symmetric (what means {tilde over (Y)}_(dec)={tilde over(Y)}_(dec) ^(T)) device is built where the non diagonal elements of theadmittance matrix can be tuned individually. This is simply done byplacing admittances −{tilde over (Y)}_(ded,m,n) from port m to n. Inmost cases (coupled coil arrays), the mutual inductivities are positivewhat results in capacitive elements in the decoupling matrix. If themutual inductivities (or equivalent coupling from different origins)become negative, than inductors are used. After decoupling, the coilelements have to become resonant again. This can be done by addingelements to ground in the Y-device or by retuning the elements bychanging the resonance capacitors.

The transmission lines described above have additional advantages:

(1) The decoupling can be placed anywhere, no complicated links have tobe build inside the coil.

(2) The coil elements can be switched off (detuned) easily andindividually.

(3) The transmission lines can be used to match the coil to theimpedance of the feeding system.

The detuning can be solved by simply adding switchable shorts at theindividual ends of the transmission lines. The short near the decouplingelements is transformed into an open circuit inside the coil elements.This also works individually, e.g. each element can be switched offwhile others are still in use.

It is further more possible to match the coil with the decoupling systemalso by choosing the line impedances (individually) to asZ₀=√(Z_(match)*R_(loss)). In this case, the coil can be fed directly atthe decoupling circuit. Alternatively, matching can be done anywhere onthe elements in a traditional way. A smaller impedance of thetransmission lines can be advantageous and can be realized by connectingsome lines in parallel. The different line impedances do not affect thepossibility to decouple and detune individually.

Generally, the reactive elements 120, 122, 124 of the compensationnetwork 42 can be realized in many ways as long as the non diagonalelement of the corresponding Y-Matrix is chosen by the value thatenables decoupling. In general, lumped capacitors or inductors will bethe best choice.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be constructed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A radio frequency (RF) coil system compatible with magnetic resonance(MR) imaging, the RF coil system comprising: a coil including aplurality of coil elements, wherein the coil is configured to perform atleast one of transmitting radio frequency excitation pulses into anexamination region and receiving responsive radio frequency pulses fromthe examination region; and a compensation network including: a reactivenetwork comprising a plurality of reactive elements including at leastone of capacitive elements and inductive elements, the reactive elementsof the reactive network having capacitive or inductive values such thatthe compensation network decouples each coil element from other coilelements, and decoupling segments each having an effective quarterwavelength (λ/4) at a resonance frequency of the coil element, whereineach decoupling segment includes (i) an RF conductor which iselectrically coupled at a first end to a first connection point of anassociated coil element of the coil and which is electrically coupled ata second end to the reactive network and (ii) a shield conductor whichis electrically coupled at one end to a second connection point of theassociated coil element and to a ground point such that the reactivenetwork is separated from the coil elements by the decoupling segments.2. The RF coil system as set forth in claim 1, wherein the reactivenetwork includes: at least reactive elements electrically coupledbetween neighboring coil elements in order to at least individuallydecouple the coil elements from one another.
 3. The RF coil system asset forth in claim 2, wherein the reactive network further includes:reactive elements electrically coupled between pairs of non-neighboringcoil elements in order to decouple the non-neighboring coil elementsfrom each other.
 4. The RF coil system as set forth in claim 1, whereinthe reactive network includes: reactive elements electrically connectedto the second connection point of each conductor in order to at leastdecouple pairs of coil elements.
 5. The RF coil system as set forth inclaim 1, wherein each coil element of the RF coil includes: a first anda second portion, which first portion is coupled to a first end ring anda conductor of an associated decoupling segment and which second portionis coupled to a second end ring and a shield conductor of the associateddecoupling segment.
 6. The RF coil system as set forth in claim 5,wherein each end ring includes: corresponding capacitors disposedbetween neighboring coil elements, and each coil element includesresonance capacitors, the capacitors cooperate with the reactive networkin order to decouple the neighboring elements from one another.
 7. TheRF coil system as set forth in claim 1, wherein the coil includes atleast one of a TEM coil, a birdcage coil, and a surface coil array asthe coil.
 8. The RF coil system as set forth in claim 1, wherein theplurality of coil elements comprise: a plurality of loop coil elementsas the coil elements.
 9. A magnetic resonance imaging system comprising:a main magnet configured for generating a substantially temporallyconstant main magnetic field in the examination region; magnetic fieldgradient coils configured to impose selected magnetic field gradients onthe main magnetic field within the examination region; and the RF coilsystem as set forth in claim
 1. 10. A magnetic resonance methodcomprising: generating a substantially temporally constant magneticfield in the examination region; and with the RF coil system as setforth in claim 1, conducting a magnetic resonance sequence including atleast the step of applying RF pulses to the coil elements; and receivingmagnetic resonance signals.
 11. The method as set forth in claim 10,wherein the coil elements extend in a direction substantially parallelto the substantially temporally constant magnetic field and the methodfurther includes: adjusting the reactive elements of the reactivenetwork in order to decouple the coil elements from each other.
 12. Themethod as set forth in claim 11, further including: selectively shortingthe conductor and the shield conductor in order to detune an associatedcoil element during generation of radio frequency pulses.
 13. A magneticresonance system including: a main magnet which generates a mainmagnetic field through an examination region; a plurality of radiofrequency (RF) transmitters which generates RF resonance excitationpulses at a resonance frequency of selected dipoles in the examinationregion; a plurality of RF receivers which receives and demodulatesresonance signals from dipoles in the examination region; a plurality ofRF coil elements connected with the RF transmitters and disposedadjacent the examination region; a plurality of effective quarterwavelength (λ/4)cables, which each cable includes (i) an RF cableconductor connected at one end to a first connection point of anassociated one or the coil elements and (ii) a shield conductorconnected at one end to a second connection point of the associated oneof the coil elements and also to a ground point; and a reactive networkelectrically coupled to the second ends of the RF conductors of theeffective λ/4 cables such that the reactive network is separated fromthe coil elements by the effective λ/4 cables, the reactive networkcomprising a plurality of reactive elements including at least one ofcapacitive elements and inductive elements, the reactive elements of thereactive network having capacitive or inductive values determined todecouple the coil element from each other.
 14. The system as set forthin claim 13, wherein the effective λ/4 cables include coaxial cables inwhich the shield conductors surround the RF conductors as a shield. 15.The system as set forth in claim 13, wherein each coil element is drivenindependently via a transmitting line which is coupled to acorresponding effective λ/4 cable and one of the transmitters in orderto selectively apply RF pulses to an examination region and one of thereceivers in order to receive the responsive RF pulses, wherein thecables match at least one impedance of an associated coil element to theimpedance of the transmitting line.